The need arises in the production lines and processing lines of metals such as steel and copper sheeting to maintain a strip of moving steel sheeting, copper sheeting, or the like in a specific center position and edge position. For instance, in a furnace for continuously annealing steel sheeting, it is necessary to control the center position of the moving steel sheet so as to substantially match the center position of the heat treatment furnace so that the moving steel sheet will not be damaged or broken as a result of this sheet meandering and hitting the furnace wall. Furthermore, in cases where steel or copper sheeting is wound in a coil, in order to prevent the edges of the coil from becoming uneven, the sheeting must be wound in a controlled fashion so that the edge positions of the steel or copper sheeting will be in specific positions with respect to the winding apparatus. A control apparatus called CPC (Center Position Control) or EPC (Edge Position Control) is used to achieve such control.
In CPC, generally the center position of a strip is sensed by a center position sensor, and feedback control is performed by varying the inclination of a roll called a steering roll, so that the center position of the strip remains constant. In EPC, generally the edge position of a metal strip is sensed by a strip end position sensor, and the position of the winding apparatus is moved, so that the relative positional relationship between the strip edge position and the winding apparatus is kept constant.
Optical, pneumatic, electrostatic capacitive, and electromagnetic inductive types have been used as the center position sensors and end position sensors used in such CPC and EPC. These are sensors that basically sense the edge position of a strip or the like, and if the edge positions on both sides are sensed, the center position can be ascertained as the center point between the two edge positions.
With an optical sensor, a light projector and a light receiver are disposed across from each other with a metal strip in between, and the edge position of the metal strip is sensed by measuring with the light receiver the proportion of the light from the light projector blocked by the metal strip. Another kind of optical sensor features a sensor that makes use of a camera. In cases where a camera is used to detect the position of a strip end, there is a configuration in which a light projector that emits light is disposed on the opposite side from the camera, i.e., on the opposite side with the metal strip in between, and a configuration in which a light projector is disposed on the same side as the camera, the light is shined on the metal strip, and the position of the metal strip end is determined from the reflected light.
With a pneumatic sensor, an air blower and an air pressure sensor are disposed across from each other with a metal strip in between, the air that is blown is blocked by the material being sensed, and the edge position of the metal strip is detected by utilizing the fact that the pressure sensed by the air pressure sensor varies according to the edge position of the metal strip.
With an electrostatic capacitive sensor, a metal strip is passed between two parallel plate electrodes, and the edge position of the metal strip is detected by utilizing the fact that the electrostatic capacity between the electrodes varies according to the edge position of the metal strip. With another type of sensor, an AC signal is applied between electrode plates disposed with a metal strip in between, and an electric field is generated between the electrode plates. The metal strip is generally grounded, and the strength of the electric field that is received varies according to the position of the metal strip because of an electrostatic blocking effect.
With an electromagnetic inductive sensor, generally a sending coil and a receiving coil are installed with a metal strip in between, and the edge position of the metal strip is detected by utilizing the principle that the magnetic field generated from the sending coil is attenuated by the effect of the eddy current generated on the metal strip.
Such a method and apparatus for measuring the position of a metal strip are described in Japanese Patent 2571330 (Patent Reference 1), for example.
Of these types of sensor, a problem with an optical sensor is that it is susceptible to the effects of steam, dust, and so forth, and can only be used in places where the environment conditions are relatively favorable. Furthermore, a fluorescent lamp is used as the projector because it is low in cost and easy to handle, but it has a short service life and has to be replaced periodically. In cases where a camera is used, there are also problems with soiling and the service life of the fluorescent lamp, making it difficult to use such a sensor for extended periods without maintenance.
With a pneumatic sensor, a constant air flow is required, so that there is a problem in terms of energy consumption, and another problem is poor detection precision. Therefore, such sensors are rarely used except in places where explosion-proof performance is required.
An electrostatic capacitive sensor needs to detect an extremely slight change in electrostatic capacity, on the order of a few picofarads. However, a change in the electrostatic capacity as small as this can be produced as a result of a change in the temperature of a coaxial cable or of the approach of a person to the electrodes, so that it is difficult to make a stable measurement.
With all types other than an optical sensor (camera type), the edge position is detected from the amount of attenuation of a signal caused by insertion of a metal strip between a sender and a receiver, and the amount of signal received is measured as an analog quantity. Inside a furnace such as a continuous annealing furnace, the condition of a sensor cannot be checked visually, so that the condition of the object being measured must be evaluated only from the analog signal that is output from the sensor. Accordingly, in the event of some kind of malfunction, that is, if the signal that is output from the sensor should exhibit abnormal behavior, a problem is that the cause cannot be ascertained from the analog signal alone, and it takes time to rectify this situation.
Moreover, with all these types of sensor, a metal strip has to be sandwiched in between, or the apparatus has to be installed to one side of the metal strip. Therefore, it is difficult to provide the sensor inside a heating furnace such as a continuous annealing furnace, and large-scale cooling (such as water cooling) is required. Furthermore, another problem is that the apparatus can be damaged due to damage to the metal strip or the like.